ABSTRACT: Soft tissue sarcomas are tumors of the connective tissue that account for an estimated 12,000 new cancer cases annually and carry a poor prognosis with a five year survival rate of 50% despite treatment. An important research objective in improving therapy for soft tissue sarcoma patients is to understand how genetic mutations affect soft tissue sarcoma development and radiation response. Intriguingly, next- generation sequencing data from The Cancer Genome Atlas and other massive cancer sequencing efforts have identified Alpha Thalassemia and Mental Retardation X-linked, or ATRX, as the second most frequently mutated gene in soft tissue sarcoma. ATRX is perhaps best known for its role as regulator of alternative lengthening of telomeres (ALT), a telomerase independent tumor maintenance mechanisms found in 15% of all human cancers. Interestingly, ATRX is predictive for overall survival in multiple human cancers and researchers recently demonstrated that ATRX knockdown leads to radiosensitization in glioma cell lines. Despite the clear importance of this gene in multiple human cancers and its frequent alteration in soft tissue sarcoma, the role of ATRX in soft tissue sarcoma remains relatively unstudied. The long term goal of this project is to improve the efficacy of current therapies for soft tissue sarcoma patients and enable the development of novel therapeutics for the treatment of human cancers. The overall goal of this proposal is to determine the effect of Atrx deletion on the radiosensitivity and innate immune response of soft tissue sarcoma. To achieve this, the Cre-LoxP recombinase system has been used in genetically engineered mouse models to generate the first primary mouse model of soft tissue sarcoma with ATRX deletion. The central hypothesis is that loss of ATRX impairs DNA damage repair, delays tumor development and increases radiosensitivity in soft tissue sarcoma. To test this hypothesis, primary soft tissue sarcomas with ATRX deletion will be compared to primary soft tissue sarcomas that retain Atrx. Using in vitro and in vivo model systems generated using Cre-LoxP and dual recombinase technologies, I will test this hypothesis in three specific aims: Aim 1: Investigate the effect of Atrx deletion on tumor growth and repetitive element associated mitotic dysfunction in a primary mouse model of soft tissue sarcoma Aim 2: Determine the role of ATRX in DNA damage repair and sarcoma response to ionizing radiation Aim 3: Determine the effect of Atrx deletion on radiation induced mitotic dysfunction and cGAS-STING innate immune signaling in sarcoma